Electron multiplier



NOV. 5, 1940. E, UNDER 2,220,161

ELECTRON MULTIPLIER Original Filed Nov. 30, 1935 Patented Nov. 5, 1940PATENT OFFICE ELECTRON MUIJTIPLIER Ernest G. Linder, Philadelphia, Pa.,assignor to Radio Corporation of of Delaware America, a corporationOriginal application November 30, 1935, Serial No. 7 52,299, now PatentNo. 2,156,264, dated May 2,

1939. Divided and t 24, 1937, Serial No. 18

Claims.

This is a division of application Serial- No. 52,299, filed November 30,1935, now Patent No. 2,156,264., May 2, 1939.

My invention relates to' circuits for electric 5 discharge devices andparticularly to circuits for devices of the type wherein amplificationof a primary electron stream, such, for example as is emitted from athermionic cathode or from a photosensitive surface exposed to light, isaccomplished through utilization of the phenomenon of secondaryemission.

If an electrode is subjected to electron bombardment it will emitsecondary electrons. The ratio of the number of secondary electrons tothe number of primary electrons depends, in part, upon the character ofthe bombarded surface, and upon the potential difference between thesurface and the source of electrons. For, example, a ratio of five ormore secondary electrons to one impinging electron is readily obtainablewith metallic surfaces treated in known ways and subjected to dischargesat potentials of 300 to 400 volts.

The use of electron-multipliers in the detection, amplification andgeneration of relatively low radio-frequency waves has been attendedwith considerable measure of success in recent years. Attempts to applyprior art tubes of the type described to ultra-short wave Work (saycentimeters), however, have either failed entirely or have resulted inunreliable and inefiicient operation either because of the largecapacitance occasioned by the very presence of the multiplyingelectrodes, or because their position with respect to the otherelectrodes has been such as to make the electron transit time of longduration.

It is, accordingly, an object of my invention to provide an improvedelectric discharge device, utilizing secondary electron emission thatshall be responsive to waves of ultra-high frequencies.

Another object of my invention is to provide an electron-multiplierwherein the inter-electrode capacitance and the electron transit timeshall be of small magnitude.

Another object of my invention is to increase the ratio of secondary toprimary electrons in electron multipliers employing secondary electrodesof a given emissive constant.

A further object of my invention is to improve the efiiciency ofultra-short wave oscillators.

Other objects willbe apparent and the invention itself both as to itsorganization and method of operation will be best understood byreferenceto the following specification and to the accompanying drawing wherein:a

Figure 1 is a perspective view-of an electronhis application December1,508

discharge device embodying my invention, the envelope of the tube beingbroken away in part in order to show the internal structure moreclearly,

Figure 2 is a cross sectional View of the electrode assembly of Fig. 1taken on the long axis of the tube of that figure and which is referredto in explaining the operation of the device,

Figure 3 is a cross sectional View of device within the invention,employing a photo-sensitive cathode and a unitary anode,

Figure 4 is a circuit diagram illustrating one manner of connecting thedevice for the generation of oscillations of ultra-high frequencies, and

Figure 5 is a circuit diagram illustrating one manner of connecting thedevice for amplifying oscillations of ultra-high frequencies.

In carrying my invention into efiect, I may employ a tube similar inmany of its constructional details to that disclosed in my application,Serial No. 703,346, filed December 21, 1932,.- Such .a device isillustrated in Fig. 1, which showsa magnetically controlled tube of thesplit-anode type which comprises a highly evacuated envelope I having apress 3 for supporting certain of the electrodes. Two anode plates 5 andi are pro- Vided, each plate being in the form of a half cylinder.Anodes 5 and I are supported from conducting rods 9 and II which aresealed into the end of the envelope I opposite the press 3, these rodsforming a transmission line for con.- ducting energy outside the tubeenvelope.

Additional support for the anodes 5 and l is provided in the form of arod 6 which mechanically and electrically connects theconductors 9 and Hat points which are nodal points on the said conductors when the tube isoperated at the high frequency for which it is designed- The rod 6 issupported at the middle from a supporting rod -8 by means of a rod I9and glass bead 12. In addition to functioning as a support for theanodes, the rod 6 acts as a short circuit between them for oscillationshaving a lower frequency than the desired frequency.

The cathode I3 consists of a straight ther- 45 mionic filament which isco-axially positioned with respect to the anode plates 5 and I andsupported at each end by conductors l5 and IT sealed into'the press 3.

A strong magnetic field of constant intensity 50 is provided by means ofmagnet coils l9 which, preferably, are so positioned that the lines offorce of the magnetic field are parallel to the axis of the anode platesand filament. It should be understood, however, that the lines of force55 UKUDD ncr EREHUL may be at an angle with respect to the axis of theanode plates.

In accordance with my invention, I provide electro-static end-plates 2|and 23 having their inner surfaces suitably treated as by theapplication of caesium or equivalent electron-emissive substance torender them secondarily-emissive. These plates 2| and 23 are positionedat opposite ends of the anodes 5 and 1 in any suitable man? ner as bymeans of rods and 21, respectively, extending from the press 3. Thesesecondarily emissive end-plates are maintained at a positive potentialwith respect to the filament l3. general, the two electrostatic plateswill be maintained at the same potential, although insome cases it maybe desirable to. give them a certain potential difference.

The operation of my improved electron-multi; plier will be understood byreference to Fig}, which is a cross sectional View on an enlarged scaleof the electrodes shown in Fig. 1. The approximate distribution of the-electr'ostatic lines of force is here indicated, this distributionbeing that of the electrostatic field which is in the plane of thepaper. In order to simplify the. drawing, the lines of force are notindicated in, the upper left-hand and the lower right-hand portions ofthe figure. In this figure, as in Fig, 1, the filament is indicated atl3, the anode at l and the secondarily emissive end-plates at 2| and 23.I

The spiral dotted lines 31. runningin an upward direction parallel't'othe filament, andthose designated ,33, running in similar paths but inthe opposite'direction show typical primary electron paths when themagnetic and electrostatic fields are of an intensity such that thedevice is operating at its highest efficiency. Under these conditionsthe primary electrons 3|, 33.e1nanating from the filament travel inspiral paths toward the nearest end plate; The limit of the path in adirection normal to the filament is determined by the adjustment of themagnetic field and to some extentbythe potential applied to the anode.no case is the intensity of this field or the anode potential such as topermit the primary electrons to graze. or otherwise contact the anode i.As indicated, the primary electrons are accelerated toward 'theendpla'tesby the electrostatic field generated by these electrodes sothat in' impinging thereagainst they release secondary or impactelectrons 3l'3 3 from these emissive surfaces. The secondary-electrons3l'33; so released, are in turn'attracted to the more positively chargedanode l. In their travel to this terminal electrode they toocircumscribe curved paths.

One very real advantage of e'lectr'o'n multipliers constructed inaccordance with the principles of my invention is this: theratio ofsecondary electrons to primary electrons is greater than that obtainingin devices constructed and operated in accordance with the prior art.This'is so because the vast majority of the primary electrons (3I-33)necessarily strike the secondarily emissive electrodes 2 l-23 at anacute angle whereby they ricochet or graze these surfaces releasing moresecondary-electrons than would be the case if they struck these surfacesin a direction normal thereto. Incidence at an acute angle-resultsingreater efliciency for high velocity electrons because the penetrationinto the metal is less whereby the secondary-electrons escape morereadily.

The described phenomenon is especially ad'- vantageous where the tube isemployed in ultrahigh frequency work for under such conditions it is notalways practical to incorporate a plurality of successive multiplyingstages in a single envelope as is the practice in tubes designed to dealwith lower frequencies.

It is not to be inferred from-the foregoing description that myinvention is to be limited to the precise embodiment illustrated byFig. 1. Quite obviously, a unitary anode may be substituted for thebi-part electrode 5-4 of this figure and the di pole conductors 9 and Hdispensed with. Further, as shown in Fig. 3, I may utilize aphoto-sensitive electrode 53 as the source of primary-electrons insteadof the thermionic filament l3 of Fig, 1. In this cross sectional view 7of a structure embodying my invention the electrodes are symmetricallydisposed about a central axis of a transparent envelope i; thephotosensitive primary source 53 defining this axis is completelysurrounded by a unitary open ended cylindrical anode 55. This anode 55is of wire mesh or otherl-foraminous construction to permit light (whichmaybe eithersteady or fluctuating in character) from an external source,not shown, to impinge upon the light sensitive cathode 53. A reflector65 is preferably provided for directing the light rays, indicated by thearrows, over the entire surface of the cathode. An end plate tlispositioned adjacent, but not touching, each open end of the anode, asdescribed in connection with Fig. l the inner surfaces of theseelectrodes are treated with caesium or other suitable material to renderthem secondarily-emis- SlVe.

Fig. 4 shows diagrammatically a device within my invention arranged as agenerator of ultrahigh frequency oscillations. The anode 55a, which mayhere be assumed to be of unitary construction, is not an outputelectrode but serves to accelerate theprimary electrons and to collectthe secondary electrons from the emissive elec trodes; Use is here madeof the negative resistancecharacteristic which obtains in the end platecircuit by reason of the secondary-emissive characteristicof theseelectrodes 61a, 63a. A tuned circuit=L-C is connected between the endplates and the output taken ofi by a suitable coupling devicesymbollically indicated by the coil 0. P. As indicated by the arrowsuperimposed upon magnetic coil 19 and by the potentiometer Pl acrossthe energizing source Bl for the end plates the intensity oftheelectromagnetic and of the electrostatic fields may be adjusted toensure optimum performance.

Fig. 5 shows diagrammatically a device within my invention arranged asan amplifier of ultra high frequency oscillations which are impressed asby means of a transformer Tl upon the tuned anode circuit L! Cl. Herethe electrostatic and magnetic fields are so adjusted that the electronsrevolve in their orbits at a frequency equal to that of the oscillationsimpressed upon the anode 5512. When the frequencies are equal resonanceis achieved between electron motion and the applied oscillatoryvoltages, this results in absorption of eg bythe electrons so that theystrike the fin-1513,1788 BI?) and 631) with increased velocity therebyejecting'more secondary electrons. As a result of the increasedsecondary-emission the steepness. of the negative resistancecharacteristic is increased whereby the oscillations appearing in thetuned output circuit L2 C2 are of increased amplitude.

Various structural and circuit modifications DLHHUH [XUUM willsuggest'themselves to those skilledin the art.

It is to be understood, therefore, that the foregoing is to beinterpreted in an illustrative and not in a limiting sense except asrequired by the prior art and by the appended claims.

I claim as my invention:

1. The combination with an electron multiplier comprising a source ofprimary electrons, a pair of secondary-electron emitter electrodes and apair of collector electrodes, means for directing primary-electrons fromsaid source to said pair of emitter electrodes and for directingsecondaryelectrons from said emitter electrodes to said pair ofcollector electrodes, a tuned circuit connected between said collectorelectrodes for impressing a radio frequency voltage thereon and a secondcircuit adapted to be tuned to the said frequency, connected betweensaid emitter electrodes.

2. In combination, an electric discharge device comprising a cathode,and an anode disposed about a common axis, a pair of secondary-electronemissive electrodes disposed adjacent opposite ends of said anode inplanes normal to said axis, means for establishing an electrostaticfield in the space between said end plates, means for inducing amagnetic field in said space, means for adjusting the relative directionand intensity of said electrostatic and magnetic fields to cause primaryelectrons from said cathode to travel in spiral paths to said end platesand to release secondary electrons therefrom by impact excitation, andmeans for maintaining said anode electrically positive with respect tosaid end plates whereby said secondary electrons are drawn thereto.

3. In combination, an electric discharge device comprising a cathode andan anode disposed about a common axis, a pair of secondary-electronemissive electrodes disposed adjacent opposite ends of said anode inplanes normal to said axis, means for establishing an electrostaticfield in the space between said end plates, means for inducing amagnetic field in said space, means for adjusting the relative directionand relative intensity of said electrostatic and magnetic fields tocause I l 1 l primary electrons from said cathode to impinge upon saidend plates and to cause secondary electrons from said end plates totravel to said anode, and a tuned circuit connected between said endplates for utilizing the emission of secondary-electrons from said endplates to establish electrical oscillations in said tuned circuit.

4. In combination, an electric discharge device comprising a cathode anda blpart anode disposed about a common axis, a pair of secondaryelectron emissive electrodes disposed adjacent opposite ends of saidanode in planes normal to said axis, a tuned circuit for impressing aradio frequency voltage between the parts of said anode, a circuit tunedto said radio frequency connected between said end plates, means forestablishing an electrostatic field in the space between said endplates, means for inducing a magnetic field in said space, means foradjusting the relative direction and intensity of said electrostatic andmagnetic fields to cause primary electrons from said cathode to travelin spiral orbits to said end plates at an orbital frequencysubstantially equal to that of the radio frequency oscillationsimpressed upon said anode whereby said primary-electrons strike said endplates with velocity such as to ensure the release of the optimumquantity of secondaryelectrons therefrom, and whereby thesecondaryelectrons emitted from said end plates and flowing in the saidanode circuit will vary in accordance with the frequency impressedthereon.

5. Method of operating a magnetron having a pair of end plates whichcomprises directing the primary electrons from the cathode in spiralorbits to the end plates at a desired orbital frequency whereby theprimary electrons impinge upon the end plates and release secondaryelectrons therefrom, and drawing the secondary-electrons to the anodewhereby said secondary-electrons produce oscillatory currents of afrequency corresponding to said orbital frequency.

ERNEST G. LENDER.

